In air-conditioning systems such as are used, for example in hospitals, schools, office, apartment and other buildings it is common to pass recirculated water over the surfaces of evaporative coolers. Similar equipment is found in many processing plants including those in the chemical, paper, textile, mining and other industries.
Make-up water typically contains dissolved chemicals which become progressively more concentrated in the recirculating water due to evaporative losses. This is especially the case in air conditioning systems and cooling circuits. Corrosion of the plant including pumps, pipes, tanks, heat exchangers, evaporative coolers etc, is a major problem and commonly the pH of the water rises. The corrosion problem is usually addressed by the addition of various water treatment chemicals. The present state of the corrosion inhibition art is summarised by Hartwick, D. in ASHRAE Journal, Feb. 2001. The primary corrosion Inhibitors may be classified as being (1) reducing Agents, (2) oxidizing agents, or (3) film formers. Reducing agents are rarely used nowadays because of their drawbacks. Oxidizing agents (e.g. chromate, molybdate, nitrite) react directly with the metal surface. While chromate and molybdate are effective, they are now seldom used because of environmental and health concerns and in many States their use is banned. Nitrite in too low a concentration can cause severe pitting, and too little nitrite is worse than none at all because it will speed up the corrosion process. Exposure of nitrite to bacteria has the potential to oxidize nitrite to nitrate or reduce it to ammonia both of which can reduce the nitrite concentration with deleterious results. Attempting to control biological activity with oxidizing biocides will oxidize the nitrite to nitrate and the efficacy of non-oxidizing biocides tends to be less certain. Consequently use of nitrites has fallen into disfavour. Among the film formers ortho-phosphate and organic phosphonates are the most common inhibitors. They act by forming a protective film on metal surfaces, but suffers from a tendency to precipitate with metal ions or hardness salts in the bulk water.
A second problem arises from the existence of slimes, bio-film, bacteria and fungi in waters. Slime and bio-film reduce pump efficiency and may seriously interfere with flow rates. In addition slimes reduce heat transfer across heat exchange surfaces, blind filters, and plug nozzles. The presence of slime and bio-film promotes corrosion because sessile bacteria in the slime or bio-film release acids and because the slime and bio-film adsorb and reduce the effectiveness of other water treatment chemicals. The term “slime” refers to a broad range of mucous, viscous, and leathery materials. These materials typically comprise or originate from polymeric, generally polysaccharide excretions produced by a broad spectrum of micro-organisms. In the past and up to the present biological deposits of all types including slime and bio-film are treated by the addition of biocides. Where slime and bio-film are present, biocides are frequently added in an effort to destroy the bacteria or microflora population which may produce the slimes. Chemicals which are used for this purpose included chlorine compounds such as chlorophenates; organomercurial compounds such as phenylmercuric acids; thiocarbamate compounds; thiocyanate compounds such as the isothiocyanates and methylene-bis-thiocyanate (MBT); tributyltin oxide; and the like. However, these chemicals are costly and highly toxic in the quantities known to be required for effective control of microbial populations. The possibility of their release into the environment is unacceptable, and their removal from water prior to disposal is uneconomical and poses risks of environmental pollution. Environment and occupational health and safety regulations now prevent the use of many such biocides in water treatment systems. Additionally, it appears that no precise correlation exists between the size of the bacterial population and the accumulation of slime or bio-film. Substantial slime accumulations have been observed even in waters having a low bacterial count. Similarly, high bacterial counts have been observed in waters having no significant slime accumulation. Consequently, use of a biocide may not adequately control biological slime or bio-film accumulations.
A further problem arises from the presence of planktonic bacteria in air-conditioning and some other systems, and especially from bacteria harmful to humans such as Legionella. First discovered in 1976, Legionella has the unusual characteristic of causing two diseases—Legionnaires disease and Pontiac Fever. Legionnaires disease is a pneumonia which affects 2-5% of those exposed. Between 5-15% of those who contract the disease die from it. Pontiac Fever attacks 95% of those exposed. Planktonic bacteria exist as a suspension in the bulk water. Planktonic Legionella bacteria may be carried by air bome spray particles from the system. Legionella pneumophilia bacteria are pathogenic when inhaled after the water in which they are resident becomes atomised. They may infect, or indeed ultimately kill, persons in the vicinity. It is therefore important to keep bacterial levels below acceptable limits. No biocide at a level safe to use in an air-conditioning system is effective to kill both sessile and planktonic Legionella in an operating system.
Legionella breeds in bio-film and slime and it is widely believed that that the best means of control is to close down a plant periodically for removal of the slime and bio-film eg by physical scrubbing and then treating with sodium hypochlorite to disinfect its surfaces. In fact, the Law in the State of New South Wales requires such action in water cooling towers at intervals of no longer than 3 months, and many other states have, or propose, similar legislation.
In the last decade, as an alternative to treatment with biocides, it has been proposed that slime and bio-film accumulation be controlled by use of enzymes. The various proposals for slime and bio-film control using one or more enzymes can be classified into two main groups. The first group consists of enzyme treatments including one or more protease enzymes, and the second to enzyme systems having one or more enzymes but not including a protease. The enzymes specifically attack the slime layer surrounding sessile bacteria but have little effect on planktonic Bacteria. Much of the work conducted with enzymes has been directed primarily at paper production where conditions are excellent for growing slime and bio-film, where the damage to production from slime and bio-film is costly, and where corrosion is a relatively minor problem so that other water treatment chemicals are not a complicating factor. The emphasis in such systems is on slime and bio-film elimination. Bacteria are only a problem in paper making insofar as they produce more slime, and as there is no correlation of slime production with the presence of planktonic bacteria, it is sufficient to prevent sessile micro-organism multiplication.
In contrast, the present inventor has found have found that in cooling towers the presence of an enzyme can result in an increase in planktonic Legionella concentration. It is believed this is partly because the enzyme physically releases micro-organisms trapped in the bio-film, and also partly because a large number of Legionella organisms may be engulfed within protozoa and thereby protected from the enzyme while the protozoa is released from the slime or bio-film . Experiments have shown that some species of Legionella can multiply intra-cellularly within certain free living protozoa. This may well be a hitherto unrecognised problem in paper mills and possibly elsewhere.
A similar problem to that occurring in recirculating water systems occurs in tepid water systems such as are found in hospitals, nursing homes, schools, jails and hotels to provide “hot” water at a “safe” temperature to showers, wash-basins or in spa baths. For almost thirty years it has been known that Legionella species can colonise such tepid water systems. Legionella species have adapted themselves to survive temperatures of up to approximately 55° C., conditions which kill many competing organisms. Legionella pneumophilia bacteria are pathogenic. At least one death has been attributed to infection with Legionella contracted from inhalation of the water spray of a shower in a hospital tepid water system. Hitherto the only treatment recommended for tepid water systems has been to periodically flush with sodium hypochlorite. The present inventors have found that chlorine treatment is ineffective against sessile Legionella harboured in a biofilm in the interior of a tepid water system.
Any discussion of the prior art throughout the specification should in no way be considered as an admission of the state of the common general knowledge in the field. Although the present invention is discussed with particular reference to Legionella it will be understood that similar considerations apply to many other micro-organisms including other harmful bacteria, fungi, moulds, etc. In water treatment systems the bacteria of general concern are gram positive.
It has also been proposed to treat water with a combination of an enzyme and a biocide. U.S. Pat. No. 4,684,469 (Pederson) describes a method to increase the antimicrobial activity of a biocide in an industrial water stream in which a selected biocide is combined with a polysaccharide degrading enzyme. Pederson used enzymes generated in-situ by bacterial cultures which are themselves pathogenic and unsuitable for use in industrial water systems. His preferred biocides are MBT and dithiocarbamates both of which are undesirable from an environmental and occupational health viewpoint. The preferred enzyme is a levan hydrolase. Pederson requires that the concentration of enzyme be monitored to maintain a concentration of at least 2 ppm of a preparation having an activity of at least 500 u/ml. However monitoring can not be done automatically and requires time consuming and labour intensive laboratory analysis. The examples given by Pederson show that under laboratory conditions the enzyme alone has substantially zero (or a negative) effect on microbial colony formation. When biocide was added half an hour after enzyme addition, the reduction in colony formation of the combination was greater than the reduction obtained by the biocide alone by a factor of 3 log or 4 log. U.S. Pat. No. 5,324,432 (Robertson) noted that Levan Hydrolase had no effect on sheathed micro-organisms such as are found in paper mill systems and proposed to treat with a protease and a biocide. The preferred combination is DBNPA and trypsin. The only example given is “contemplative” and involves treatment of the water with enzymes for 1-15 minutes followed by addition of hypochlorite.
These proposals prefer to add the biocide separately from, and subsequently to, addition of an enzyme to the papermaking machine's water stream. However simultaneous separate addition was considered feasible.
The present applicant has found combinations of biocide/enzyme as proposed in the prior art are not effective in cooling towers. At first it was hypothesized that this may be due to deactivation of the enzymes by the biocides. The present inventor has found that in systems of the kind described by Pederson and Robertson the enzyme is virtually ineffective within two hours of addition of the biocide. Thus even in a favourable case the enzyme must be added continuously or repetitively at short (less than 2 hour, preferably less than 30 min) intervals. This renders the system impracticable because of the difficulty of monitoring enzyme activity in a cooling tower environment and because of the low effectiveness and high cost over time. However the present inventor has also found that in the presence of corrosion inhibitors, this problem of enzyme denaturation is greatly exacerbated making use of enzyme/biocide combinations impractical in cooling towers and too expensive to be considered.
In summary, the present inventor has found that corrosion inhibitors in practical use in cooling towers nowadays de-activate enzymes either via direct oxidation or via surface absorption. In addition, enzymes are not compatible with most biocides which absorb onto enzyme proteins thus effectively deactivating enzymatic activity. Although some enzyme manufacturers claim that enzyme maintain their activity in a chlorine environment, the claim relates to laundry conditions and exposure times of around 10-15 minutes, as opposed to at least 24 hours as required for cooling tower maintenance.
To date no treatment has proved to be capable of avoiding the need for closure of cooling towers at 3 monthly intervals for cleaning. Enzymes are not effective in systems treated with preferred modern primary corrosion inhibitors (i.e. oxidizing agents or film formers). Biocides, at levels deemed safe to use, are not effective because they cannot penetrate through bio-film and attack Legionella in the bio-film or the Legionella that is parasitic within protozoa. Moreover, biocides are unable to kill both sessile and planktonic bacteria at levels of biocide considered safe. Combinations of enzymes and biocides result in each deactivating the other and have been found to be ineffective in the presence of modern corrosion inhibitors which rapidly deactivate enzymes.
No enzyme system, biocide, or enzyme/biocide combination from the prior art has been found which simultaneously meets the requirements for:
(1) compatibility with corrosion inhibitors,
(2) environmental acceptability,
(3) health and safety acceptability,
(4) ability to control bio-film containing Legionella and ability to control planktonic Legionella sufficiently so that closure of the plant for cleaning at three monthly intervals can be avoided.
In addition, in the prior art when an enzyme and a biocide have been combined it has usually been necessary to add them separately. That is because the biocides have tended deactivate the enzymes or the enzymes to deactivate the biocides. This separation necessitates duplication of pumps, storage and feed tanks, as well as ancillary equipment for stirring, feed control and the like. It would be very advantageous to provide a water treatment composition which met all the requirements for treating water in an air-conditioning system or any other bulk water system and which could be combined in or delivered from a single container or tank. More preferably the combination would be available as a storage stable composition or concentrate.
It is an object of the invention to provide a method of treatment of industrial recirculating water which avoids or ameliorates at least some of the above discussed disadvantages of prior art. It is an object of preferred embodiments of the invention to provide a method and composition which will avoid the need for plant closure, or at least prolong the period in which the plant can be safely operated without closure for cleaning.
It is a further object to provide a method and composition for remediation of tepid water systems in which a biofilm harbours micro-organisms, such as for example, Legionella.